This invention relates to cathodic protection systems for preventing corrosion of a metal structure, and more particularly, to a method and apparatus for remotely determining the functional condition of a cathodic protection system.
Cathodic protection systems are employed to prevent corrosion of metal structures exposed to an electrolytic environment. Cathodic protection can be effected for submarine or subterranean corrodible structures by electrically connecting the corrodible structure to sacrificial anodes constructed of a metal that is higher in the electromotive series than the protected structure, i.e., a metal that is anodic to the material of the protected structure. When the protected structure and the electrically connected sacrificial anode are both disposed within the same electrolytic environment (e.g., earth or water containing free positive ions), a galvanic cell is formed in which the protected structure is the cathode.
Metal atoms on the exposed surface of the sacrificial anode are ionized by the surrounding electrolyte and go into solution with the electrolyte, thereby corroding the sacrificial anode. Due to the difference in electrical potential between the cathodically protected metal and the sacrificial anode, electrons produced by the electro-chemical corrosion reaction of the anode flow as an electrical current through the electrical connection between the sacrificial anode and the protected structure. When the electrons reach the protected structure, they either combine with positive ions in the electrolyte at the surface of the protected structure, or flow back to the sacrificial anode through the electrolyte to complete a current path between the sacrificial anode and the protected structure. The protected structure does not corrode since free electrons are readily available at the surface of that structure to chemically reduce or neutralize positive ions that reach the surface of the protected structure, which positive ions would otherwise initiate a corrosion reaction at the surface of the protected structure.
Often, the function of a cathodic protection system is supplemented by applying a protective coating to the exterior of the cathodically protected structures to reduce the exposure of the protected structure to the electrolyte environment. However, a protective coating will not completely isolate the protected structure from the electrolyte since small cracks or discontinuities in the coating develop as the coating ages, allowing the portion of the structure exposed through the cracks to be corroded. Further, such a coating is incapable of perfectly isolating the corrodible structure from positive ions in the surrounding electrolyte as some of the positive ions are capable of diffusion or migration through the protective coating itself.
Cathodic protection systems are capable of protecting the corrodible structure from corrosion as long as a sufficient amount of sacrificial anode remains to supply electrons to the protected structure. When an anode is nearly completely corroded, it must be replaced in order for the cathodic protection system to continue its function. The corrosion rate of the sacrificial anode, and thus the point in time when the anode needs to be replaced, is difficult to predict since it is influenced by a number of variable factors such as the composition of the surrounding soil or water and localized variations in that composition.
Although the prior art has devised a number of schemes for determining the condition of a cathodic protection system to ascertain whether the system is functioning, and/or for determining when the sacrificial anodes are in need of replacement, these schemes have proven unsatisfactory in certain applications. For example, in order to cathodically protect a considerable length of subterranean or submarine conduit or pipe, it is necessary to provide either a plurality of sacrificial anodes electrically connected to the pipe and spaced along the length of the pipe, or to provide a continuous sacrificial anode disposed along the length of the pipe having a plurality of electrical connections between the anode and the pipe with the electrical connections spaced along the pipe length. The condition of such a cathodic protection system is conventionally monitored by determining the polarity and/or magnitude of the electrical potential of the sacrificial anode and/or the electrical potential of the protected pipe with respect to a reference half-cell disposed in the electrolytic media surrounding the pipe. These determinations must be made at a plurality of locations along the length of the pipe to determine the condition of the entire system. To facilitate the monitoring tests, electrical connections, in the form of an insulated electrical conductor electrically connected to the sacrificial anode and/or the cathodically protected structure and routed to the surface of the electrolyte in which the system is disposed, are provided at various points along the pipe, e.g. at each connection between a sacrificial anode and the cathodically protected pipe.
Such prior art systems may be satisfactory under certain conditions. However, the installation and maintenance of the additional electrical connections between the anode and/or pipe and the surface of the electrolyte are expensive. Further, such monitoring techniques generally require a periodic manual test at each of the test locations to determine the local conditions of the protection system at various points along the length of the pipe, requiring maintenance personnel to traverse the pipeline route. In addition, pipelines that are routed over rough or mountainous terrain or under water are not readily accessible by maintenance personnel. Moreover, during inclement weather or during certain seasons of the year in northern areas of the country, it may be physically impossible to traverse the pipeline route and make the necessary measurements at all the test locations.
Accordingly, it is a broad object of this invention to provide apparatus and methods for monitoring the condition of a cathodic protection system for a length of corrodible structure such as a pipe or conduit that require neither electrical connections to the surface of the electrolyte nor manual tests at each connection between the sacrificial anodes and the protected structure to determine the condition of the system. It is a further object of this invention to provide methods and apparatus for remotely determining the condition of a cathodic protection system.